Camera response curve

The camera response curve is
a curve showing the relation between amount of incoming light and image pixel values of a digital camera.

This curve is sometimes also called Opto-Electrical Transfer Function.

Although optical sensors work quite linear (a certain amount of photons coming in causes a proportional amount of electrons in the sensor cell), the resulting pixel values are not proportional to the amount of incoming light. This might have several reasons:

A white balance is applied to compensate for different light temperature

While the first reason applies to all kinds of images, the other ones apply to non-RAW (JPEG, TIFF) images only. RAW images should contain linear sensor data.

The camera response curve is an issue mainly in HDR generation where it is necessary to have linear (proportional to light) input data. Non-linear data (= a badly estimated camera response curve) will result in banding in contrast gradients (f.e. around bright lights).